Some have argued that fluoridation is a risk to health, but the report by public Health England looked at rates of hip fractures, kidney stones, cancers, Down's syndrome births and all-cause mortality and concluded that there is "no evidence of harm to health in fluoridated areas".

Yet, as with so many public health interventions, the sound and fury of opposing ideology often trumps rational analysis. If the backlash against fluoridation in the Republic of Ireland is anything to go by, Public Health England can expect vigorous opposition.

The current incarnation of the opposition relies heavily on a report by self-proclaimed “fluoridation scientist” Declan Waugh, who blames fluoride for a range of illnesses. The report has been roundly dismissed by the Irish Expert Board on Fluoridation and Health, its chairman Dr Seamus O'Hickey concluding that"… in spite of its presentation, its content is decidedly unscientific … the allegations of ill health effects are based on a misreading of laboratory experiments and human health studies, and also on an unfounded personal theory of the author’s.”

Despite this, clever use of social media and strong lobbying has gained fluoridation naysayers considerable political traction, prompting the Irish government to promise yet another full review of the practice.

This kind of irrational thinking is equally apparent in the anti-vaccination movement, for example among those who continue to believe that the MMR vaccine causes autism. A 2011 editorial in the New England Journal of Medicine noted wearily that “… the spectrum of anti-vaccinationists ranges from people who are simply ignorant about science … to a radical fringe element who use deliberate mistruths, intimidation, falsified data, and threats of violence in efforts to prevent the use of vaccines and to silence critics.”

That such beliefs persist in the face of strong evidence may be a quirk of human psychology. Campaigners may see themselves as enlightened crusaders, so when their assertions are questioned or contradicted by the data, this is viewed not as a useful correction of error but rather an attack on their identity and narrative. Conspiratorial thinking is endemic in such groups with critics being regarded as agents of some ominous interest group – big pharma is a common bogeyman – that wants to conceal the truth. This becomes a defence mechanism to protect beliefs that are incompatible with the evidence.

If all else fails, attacking the messenger may be easier than accepting that your whole raison d'être is misguided.

Motivated rejection of evidence is often a symptom of cognitive dissonance, a psychological phenomenon that occurs when individuals are challenged by information inconsistent with their beliefs. They may reject unwelcome information, seek confirmation from those who already share their beleaguered viewpoint, and try to convince others of the veracity of their world view. This may explain why some people proselytise even more vigorously after their beliefs have been debunked.

That humans can be irrational is hardly a revelation, but perhaps the ugliest facet of the Irish debate is how elected representatives have given such outlandish fringe assertions a sense of legitimacy. One Irish politician has claimed that fluoridation causes cancer and Down’s syndrome; others have demanded an end to the practice, parroting claims that would have taken all of three minutes on Wikipedia to expose as utter nonsense.

The Irish government's response is appeasement, and a waste of time and public money. Not only is there already an Irish body that routinely reviews the safety of fluoridation, this is a Sisyphean task because anti-fluoride groups have already reached their conclusion, and will trust no expert body unless it agrees with their assertions. Almost certainly fluoride will get yet another clean bill of health, campaigners will reject the findings and the same tedious cycle will repeat again, in much the same way parents who oppose vaccination are impervious to the scientific literature undermining their position.

It is irresponsible for politicians to show such contempt for science that they're willing to take the lead from pseudoscientists and conspiracy theorists rather than experts. Leadership should be about making the best decisions based on the data available, even on emotive issues such as fluoridation and vaccination.

As the Irish debate lumbers on, and local authorities in the UK consider fluoridation, we should demand that our elected representatives base health policy on evidence. There will undoubtedly be opposition as there has been in the past but what is crucial is that decisions are based on scientific research, not misinformation and fear. The cost of such folly is clear to anyone who remembers the human suffering in the wake of the misinformed panic over the MMR vaccine just a decade ago.